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Is pavement Quality of any Value

Over the past 50 or more years I have read about adding all sorts of waste materials to pavements or to the material beneath the pavements. Some of the materials have benefits, some are just garbage.

Research has shown that including reclaimed asphalt in new pavements has a benefit with respect to quality as well as cost. If the resulting blend of new and recycled asphalt meets the specification requirements, the pavement should be at least as good as pavements made with new asphalt. One value of the recycled material (RAP) is that the rate of oxidation of the old asphalt on the aggregate will be less than that of new asphalt on new aggregate. That is because the rate of oxidation of new asphalt on aggregate deceases with time, (except perhaps for an asphalt from one particular crude source). The RAP asphalt has already experience the rapid oxidation phase.

Recycled asphalt shingles (RAS) are now being used. At the present time I am not comfortable with that although future testing may find it works well. Shingles consist of an air blown saturate in a felt on which a filled coating asphalt is placed. The softening point of the coating is above 200° F. Past experience has shown that the presence of air blown asphalt can accelerate non-load associated cracking. I don’t know if the addition of elastomers helps with this problem or not. Non-load associated cracking occurs when the binder cannot relax thermal stresses before they reach the failure stress. Time will tell.

Pavements have been used to get rid of glass. This is a novelty as there isn’t enough glass around to have an impact. It can work, however it must be realized that glass likes water better than asphalt resulting in possible areas of water damage.

I have heard that some agencies are adding reclaimed oil to asphalt. That is a very bad idea as paraffins and asphaltenes are incompatible. Asphalt naturally contains some paraffins which are kept in solution by the aromatics and polar materials in asphalt. Loading up the asphalt with more paraffins can cause phase separation, which would be expected to cause non-load associated cracking. Before adding such oils to asphalt it might be well to read up on the research done by Rostler et al. half a century or more ago. Refineries have had corrosion problems with the addition of such oils to their crude feed.

Reclaimed tire buffings have been added to asphalt for many years with success. Truck tire buffings (natural rubber) and passenger tire buffings (SBR) will react differently. There can be a problem in QA testing. A contractor may specify that they have added a certain amount of the tire buffings but testing on a sample taken from construction may indicate that there was less than what the contractor said there was. The tire buffings would be expected to contain some processing oils which would be extracted out, and if there was natural rubber in the buffings, it might have broken down some as cis-polyisoprene (natural rubber) is not as heat stable as SBR. It would be well for the contractor to tell the owner how much of what was added would not be found.

The original specifications for asphalt and hot mix were based upon unmodified asphalt and aggregate. Experience has shown that those specifications can still be valid with the addition of certain polymers and with the addition of lime to the aggregate. However adding other materials to the pavement simply to get rid of them doesn’t mean there won’t be unforeseen consequences ever if such mixes meet specification requirements. Early non-load associated cracking is especially difficult to predict.